Hi ,
One of my DIY panels has decided to go quiet on me after only intermittant usage . Output from the other panel is much louder . I've given the panel the once over and re-shrunk the diaphragm as a precaution , also vacuumed the panel both sides , checked stator wiring continuity and swapped over the stator connections . The diaphragm EHT is the same as the other panel (shared EHT) so I know there is not an internal short . I am now concerned that the problem is the diaphragm coating , or maybe the diaphragm charge ring which consists of copper tape with conductive adhesive . What further testing can I carry out before a rebuild is in order ?

it could be the coating, or it could be serious leakage (maybe just a fingerprint at the right point).

IŽd suggest You try an indicator circuit first.
It consists of a small flash bulb (example. 90V ingnitiontreshold) paralleled with a small capacitor (foil type, ~100nF and more than 100V) and a series connected resistor of ~10meg.
Connect this indicator circuit between the output of the HV-polarizing supply and the diaphragm connection.
The number of flashes/sec indicates how ogften the diaphragm is charged. It might flash quite often at the start, and when thereŽs a leakage, but ideally flashes only every couple of seconds when leakage is low and everything ok.

Hi ,
Thanks for the advice , unfortunately I don't have a flashbulb to hand , would a neon bulb be any use ? Earlier I cleaned the sides of the panel with a cottonbud and isoprop , there was slight residue of graphite powder . At power on there was an arc on the outside edge between the stator and the middle point of the spacers which seemed odd as I'd just cleaned the thing . Cleaned again and the arc is gone . I'm beginning to suspect that the problem may be loose particles of graphite powder inside the panel causing a partial short , probably too high for my meter to pick up . I've had this problem before , although the symptoms were a slight hissing sound rather than a loss of output

Hi ,
Each bias supply is -2400V , that was the first thing I checked and this is mentioned in the original posting . This afternoon I've blasted a bit of compressed air through the panels , vacuumed and retested but the same problem exists . I think that the problem is either the diaphragm coating or a very remote possibility , the contact ring . The diaphragm contact is made with thin copper tape with conductive adhesive and unfortunately I've run out of the tape which rules out a rebuild today . This is a real pain as recently I not only built a dedicated direct drive amp but also new frames for the panels , also another dedicated amp to drive Jordan JX150 running as bass-mid. Up until then the panels worked fine . Doh !